The present invention generally relates to battery capacity predicting methods, battery units and apparatuses using a battery unit, and more particularly to a battery capacity predicting method which is suited for predicting a remaining capacity of a battery which is easily damaged by an excessive discharge, a battery unit which enables the use of such a battery capacity predicting method, and an apparatus such as a portable electronic equipment using such a battery unit.
Recently, in apparatuses such as portable electronic equipments which are typified by notebook type personal computers or lap-top computers, lithium ion (Li.sup.+) batteries or the like are more popularly used in place of nickel cadmium (NiCd) batteries, nickel metal hydrogen (NiMH) batteries or the like. Compared to the NiCd batteries, the NiMH batteries or the like, the Li.sup.+ batteries or the like are lighter in weight and have a larger capacity per unit volume. For this reason, the Li.sup.+ batteries or the like are suited for use in apparatuses which must satisfy demands such as the realization of light weight and continuous use for a long period of time.
However, the Li.sup.+ batteries or the like are easily damaged by an excessive discharge. For example, if the user erroneously subjects the Li.sup.+ batteries to an excessive discharge, the Li.sup.+ batteries suffer an irrecoverable damage. For this reason, a battery unit which includes a Li.sup.+ battery or the like has a built-in excessive discharge preventing circuit which detects cuts off the battery output when the battery voltage becomes less than a predetermined voltage, in order to prevent deterioration of the battery function due to an erroneous operation of the user.
On the other hand, in the apparatus which uses the battery unit having the built-in excessive discharge preventing circuit, it is necessary to monitor the output voltage of the battery unit and to take measures so that data destruction or the like will not occur within the apparatus before the excessive discharge preventing circuit operates. Particularly in the case of the notebook type personal computer or the like, all of data being processed will be lost if the battery dies, and thus, the data must be saved in a non-volatile recording medium such as a floppy disk in good time by recognizing the remaining capacity of the battery. Accordingly, in order to prevent such data destruction or the like, some notebook type personal computers or the like are provided with a function of indicating the remaining capacity of the battery.
FIG. 1 is a circuit diagram for explaining a conventional battery capacity predicting method. In FIG. 1, a battery unit 501 is connected to an apparatus 502 such as a notebook type personal computer. The battery unit 501 has a built-in excessive discharge preventing circuit including a circuit portion 510 and a switch 515. The circuit portion 510 includes voltage comparators 511 through 513, and an OR circuit 514. In this example, three battery cells E1 through E3 are connected in series within the battery unit 501. An output voltage of the battery cell Ei and a reference voltage e1 which indicates an excessive discharge limit voltage of the battery cell Ei are supplied to the voltage comparator 51i, where i=1, 2 and 3. The switch 515 is made up of a field effect transistor (FET).
The battery cells E1 through E3 are made of Li.sup.+ batteries, for example. The battery capacity predicting method can roughly be categorized into a method which predicts the battery capacity by subtracting the used power from the capacity of the battery, and a method which predicts the battery capacity from the output voltage of the battery. The Li.sup.+ battery has a characteristic such that the output voltage of the Li.sup.+ battery is a maximum value when fully charged and the output voltage decreases as the discharge progresses. For this reason, in this example, the latter method will be used to predict the remaining capacity of the battery.
The voltage comparators 511 through 513 output high-level signals when the output voltages of the corresponding battery cells E1 through E3 become less than or equal to the reference voltage e1. Hence, when one of the voltage comparators 511 through 513 outputs a high-level signal, the switch 515 is turned OFF in response to a high-level signal from the OR circuit 514, so as to disconnect the battery unit 501 from the apparatus 502. As a result, when the output voltage of at least one battery cell becomes less than or equal to the excessive discharge limit voltage, the output of the battery unit 501 is cut off and the excessive discharge of the battery cells E1 through E3 is prevented.
The apparatus 502 is provided with a voltage dividing circuit made up of resistors R1 and R2, a voltage measuring circuit 520 and the like. The voltage measuring circuit 520 measures the output voltage of the battery unit 501 which is obtained via the voltage dividing circuit, and predicts the remaining capacity of the battery unit 501 by comparing the measured output voltage and a predetermined reference voltage. This predetermined reference voltage is set slightly larger than three times the reference voltage e1 of the battery unit 501, by taking into account the inconsistencies among the capacities of the individual battery cells E1 through E3. When the output voltage of the battery unit 501, that is, the sum total of the output voltages of the battery cells E1 through E3, becomes less than or equal to the predetermined reference voltage, the apparatus 502 judges that the output voltage of the battery unit 501 has become close to the excessive discharge voltage and outputs an alarm to the user, for example.
The deterioration of the battery unit 501 due to the excessive discharge occurs when the output voltages of the individual battery cells E1 through E3 become less than or equal to the reference voltage e1. When the output voltages of the battery cells E1 through E3 which are connected in series within the battery unit 501 are balanced, it is unnecessary to monitor the output voltages of the individual battery cells E1 through E3. In this case, it is sufficient to monitor the sum total of the output voltages of the battery cells E1 through E3, and the excessive discharge preventing circuit may be operated when this sum total becomes less than or equal to a reference voltage.
However, in actual practice, the capacities of the individual battery cells E1 through E3 are inconsistent, and it is not unusual that the capacities of the battery cells E1 through E3 differ by approximately 10%. In addition, the differences among the capacities of the battery cells E1 through E3 become even larger as the number of charging and discharging cycles of the battery unit 501 increases, because the capacities of the battery cells E1 through E3 also become inconsistent depending on the individual deteriorations of the battery cells E1 through E3. For this reason, the built-in excessive discharge preventing circuit of the battery unit 501 monitors the output voltages of the individual battery cells E1 through E3, and the switch 515 is turned OFF when the output voltage of at least one of the battery cells E1 through E3 becomes less than or equal to the reference voltage e1, so as to cut off the output voltage of the battery unit 501 by disconnecting the battery unit 501 from the apparatus 502.
On the other hand, the apparatus 502 monitors the output voltage of the battery unit 501, that is, the sum total of the output voltages of the battery cells E1 through E3, predicts the remaining capacity of the battery unit 501 from this sum total, and detects the operation of the excessive discharge preventing circuit within the battery unit 501 before the excessive discharge preventing circuit turns OFF the switch 515. For this reason, it is necessary to set the predetermined reference voltage with a sufficient margin by taking into account the differences among the capacities of the individual battery cells E1 through E3.
But if the above described margin is too large, the apparatus 502 will judge that the end of the discharge of the battery unit 501 is near even though the actual remaining capacity of the battery unit 501 is sufficiently large. As a result, there is a problem in that the utilization efficiency of the battery unit 501 is poor. In addition, when the charging and discharging of the battery unit 501 are repeated in a state where the remaining capacity of the battery unit 501 is sufficiently large, there is a problem in that the serviceable life of the battery unit 501 is shortened by excessive charging.
On the other hand, if the above described margin is too small, the apparatus 502 continues to use the battery unit 501 even though the end of the discharge of the battery unit 501 is near. In this case, there is a problem in that the output voltage of the battery unit 501 is suddenly cut off by the excessive discharge preventing circuit during operation of the apparatus 502. When the output voltage of the battery unit 501 is suddenly cut off during operation of the apparatus 502, data destruction and the like occur within the apparatus 502, and the user may be faced with a fatal power failure.